Check your config file, it should read fbreader if the software in the bin sub is called fbreader.app
like here:
epub:@EPUB_file:1:fbreader.app,AdobeViewer.app:ICON_EPUB

Sometimes the config files name it fbreader180.app. If so remove the 180 or rename the app in the bin sub folder.

And u put the files on the internal memory not the sd card, correct ?
U unzipped the fbreader after the download ? The file in the bin sub is called fbreader.app

Report further, hopefully it works

OK, two things were incorrect for me - the config file needs to have "180" removed, so I will do that.

Yes, the files are on the internal memory

The file in the bin subfolder is not an .app file - it doesn't unzip because I am using a Macbook Pro and it says the Classic environment is no longer supported when I double click on fbreader. Perhaps I will try the Mac OS ported version of fbreader and report back.